- In epidermal keratinocytes the balance between proliferation and differentiation must be critically regulated in order to maintain the function of the skin as a protective barrier. Deregulation can lead to conditions such as skin cancer and psoriasis, a skin disorder characterized by hyperproliferation and abnormal differentiation. An understanding of the mechanisms involved in regulating normal growth and differentiation, as well as the alterations which lead to particular pathologic states, is important for developing better therapies for these conditions. 1,25-dihydroxyvitamin D3 is known to promote the differentiation of epidermal keratinocytes both in vitro and in vivo, and this compound and/or its analogs have been used successfully to treat psoriasis. However, the mechanism(s) through which 1,25-dihydroxyvitamin D3 exerts its effects are unclear. The investigators' recent data indicate that 1,25-dihydroxyvitamin D3 regulates the activity and expression of an isoform of PLD. PLD hydrolyzes membrane phospholipids to yield phosphatidic acid, which can, in turn, be dephosphorylated to generate diacylglycerol (DAG). Moreover, evidence from their laboratory and others suggests that this enzyme inhibits keratinocyte growth and promotes differentiation. Therefore, they hypothesize that PLD, via its production of phosphatidic acid and/or DAG, mediates the prodifferentiative effects of 1,25-dihydroxyvitamin D3. This proposal seeks to define the role of PLD and its lipid products in the 1,25-dihydroxyvitamin D3-mediated regulation of epidermal keratinocyte growth and differentiation. To accomplish this goal they will seek to answer several questions: 1) Does PLD activation and expression in response to 1,25-dihydroxyvitamin D3 or its structural analogs correlate with their ability to induce differentiation? As a corollary to this question, the association between keratinocyte differentiation and PLD activation and expression will also be determined. 2) Do PLD and the products of its action mediate the prodifferentiative effects of 1,25-dihydroxyvitamin D3? In these studies the roles of PLD, PLD-derived DAG and phosphatidic acid and PKC activity in 1,25-dihydroxyvitamin D3-induced differentiation will be defined. The data obtained from the proposed studies should provide critical information on the involvement of the PLD signaling system in regulating epidermal keratinocyte growth and differentiation and may suggest a new approach for the development of therapeutic agents for the treatment of psoriasis and basal and squamous carcinomas.